1. Field of the Invention
The present invention relates to a circuit board for mounting electronic parts thereon.
2. Description of the Prior Art
A technique called a TAB (Tape Automated Bonding) method has been primarily used to mount semiconductor devices, driving liquid crystal panels noted as image display apparatuses, on a circuit board. According to the TAB method, such semiconductor devices are supplied in the form of a TCP (tape carrier pack). The TCP is fitted by a FOB (Film On Board) to a circuit board that supplies a driving signal for the liquid crystal.
In FIG. 8A, a top view of a TCP for a liquid crystal driving device is shown. The liquid crystal driving TCP includes a resin film base 12a, and an IC 11 placed on an upper surface of the resin film base 12a. A predetermined number "n" of input leads L, spaced at a predetermined pitch Pl, are provided on the bottom surface of the resin film base 12a. Each of the input leads L extends from the input terminal of the IC through a predetermined hole formed in the base 12a and protrudes from the side of the base 12a. An elongating resin film 12b is further provided on the upper surface of ends of the thus protruded leads L.
Each of the input leads L has a predetermined width Wl, and is apart from the neighboring leads by a predetermined distance Il(Il=Pl-Wl). The input lead located at the right-most side in FIG. 8A is referred to as the first input lead L1. Hereafter, any one of the "n" number of input leads L is designated by a suffix "i", indicating the order counted from the first lead L1. Note that "i" is an integer greater than zero and less than "n+1". Therefore, the reference symbol for the final input lead is Ln.
As indicated by dot lines in FIG. 8A, a plurality of output leads 8 that are very similar to the input leads L but are extending in the opposite direction from the output terminals of the IC 11 are provided on the bottom surface of the resin film base 12a. The resin film base 12a, IC 11, input leads L and output leads 8 are firmly fixed by an insulating resin 10, as best shown in FIG. 6.
In FIG. 8B, a circuit board 130 according to the TAB method is shown. The circuit board 130 is provided with the same number n of copper electrodes E arranged on the upper surface of the circuit board 130 at the same pitch Pl as the input leads L of the TCP (FIG. 8A). Similarly, each of the copper electrodes E is referred to by the reference symbol E having a suffix indicating the order counted from the first lead E1 attached thereto. Therefore, the reference symbol of the final copper electrodes E arranged on the left-most side in FIG. 8B has the suffix n. Any of the copper electrodes E can be referred to by the reference symbol Ei.
The TCP and the circuit board 130 are positioned such that every input leads Li is located above corresponding electrode Ei when the TCP is mounted on the circuit board 130. Note that components mounted on the circuit board 130 other than the electrodes E are omitted from FIG. 8B for the sake of clarity.
In FIG. 8C, a cross-sectional side view of the circuit board 130, taken along a line IIXC--IIXC, of FIG. 8B with the TCP of FIG. 8A located above is shown on an enlarged scale. Every copper electrode Ei on the circuit board 130 is provided with a solder deposit D formed thereon. Moreover, an amount of the solder deposit D on each copper electrode E is maintained to be constant in order to bond the leads L and copper electrodes E in the same shape.
With reference to FIG. 7, the concept of solder-bonding thus prepared input leads L with electrodes E by using a soldering iron is described below. Note that FIG. 7 shows the cross-sectional view corresponding to FIG. 8C wherein the first three pairs of leads L1 to L3 and electrodes E1 to E3 are bonded by solder melted from the solder deposits, respectively, and three successive pairs of leads L4 to L6 and electrodes E4 to E6 are now under going the soldering operation.
Each copper electrode E is arranged on the circuit board 130 below the corresponding lead L of the TCP, as best shown in FIG. 8C. The bonding solder D of substantially the same amount and same form is deposited on each copper electrode E. Note that the direction Db is the direction in which the input leads L are arranged from the right-most side lead (first lead L1) to the left-most side lead (final lead Ln).
Then, a soldering iron 160 of oxygen free high conductivity copper and a flux pipe 140 are moved in alignment with each other in a direction Db indicated by an arrow Db. At this time, the soldering iron 160 which is heated over the melting point of the solder and flux pipe 140 are kept in a pressure contact with several leads L to press against the solder deposits D toward the electrodes E. The copper electrodes E and the leads L of the TCP are bonded by the solder in this way.
The bonding process is hence carried out while the soldering iron 160 which is heated over the melting point of the solder is moved together with the flux pipe 140, which will be described below in more detail.
While the flux pipe 140 is held in touch with the first lead L1, a flux 150 for removing an oxide film of the solder deposit D is fed to the first lead L1 from the flux pipe 14. Immediately after the flux 150 is supplied around the first lead L1 and solder deposit D1, a front end 220 of the heated soldering iron 160 comes in touch with the first solder deposit D1 on the copper electrodes E1, to thereby melt the first solder deposit D1. Thus molten first solder D1 wets an iron plating 180 at the front end 220 of the soldering iron 160 and forms a solder pool 210 between the iron plating 180 and the first copper electrode E1.
According to the movement of the solder iron 160 in the bonding direction Db, the second solder deposit D2 on the second electrode E2 is successively melted by the heated solder iron 160. Then, the solder pool 210 grows around the first and second electrodes E1 and E2. In the example shown in FIG. 7, since the solder iron 160 has the iron plated area 180 extending between both ends 220 and 230 wide enough for covering three copper electrodes E, the solder pool 210 grows between the soldering iron 160 and three electrodes E1, E2, and E3, for example, at the same time. Within the solder pool 210, the neighboring leads L are electrically short-circuited by the molten solder.
However, as the solder iron 160 moves in the direction Db, a black chromium plating 190 at the front end 220 of the soldering iron 160 contacts with the solder pool 210. When the soldering iron 160 reaches such a position where the black chromium plating 190 contacts with the solder pool 210 between the neighboring electrodes E1 and E2, the iron plated part 180 presses the fourth lead L4 against the fourth solder deposit D4. However the molten solder can not wet the black chromium plating 190 and the substrate of circuit board 130. Therefore, the solder pool 210 is divided into two portions owing to the surface tension thereof.
The first portion is kept away from the solder pool 210 covering the second and third electrodes E2 and E3 by the black chromium plating 190. The second portion is the solder pool 210 extending from the second electrode E2 to the fourth electrode E4, wherein the molten solder melted from the fourth deposit D4 by the solder iron 160 is additionally included.
In this case, the first portion of molten solder separated from the solder pool 210 wets the first electrode E1 and lead L1, and fits them closely together. Thus, the electrode E1 and lead L1 are fitted closed together and joined by the molten solder distributed between the fitted surface thereof by capillary attraction. After the joined parts cool, the molten solder solidifies to form a first soldered mount M1 wherein the lead L1 and electrode E1 are firmly bonded.
In order to secure a stable soldering quality, it is required to form the solder pool 210 sufficiently at the front end 220 of the soldering iron 160 and also provide the black chromium plating 190 at the front end 220 of the soldering iron. That is, every copper electrode E on the circuit board 130 was used for bonding with the leads L. Moreover, an amount of the solder formed on each copper electrode E was maintained to be constant in order to bond the leads L and copper electrodes E in the same shape.
FIGS. 9A, 9B, 9C, and 9D illustrate an operation for bonding the thus constructed conventional circuit board 130 with the TCP shown in FIGS. 8A, 8B, and 8C. FIG. 9A shows that the first three pairs of leads L1 to L3 and electrodes E1 to E3 are subject to the soldering operation. The solder deposits D1 to D3 formed on the electrodes E1 to E3 are melted by the heated solder iron 160 and form the solder pool 210 distributed between the iron plated surface 180 and the electrodes E1 to E3. The flux 150 is supplied to the fourth deposit D4 and the solder pool 210. The thus formed solder pool 210 is pressed by the slanted surface of the iron plated surface of the soldering iron 160 in the soldering direction Db. According to the proceeding of the soldering operation, the soldering iron 160 is fed in the direction Db so that not a negligible amount of the molten solder of the solder pool 210 is conveyed from the first electrode E1.
Since the movement of the molten solder from the preceding electrodes is accumulatively repeated as the iron 160 moves, a smaller amount of molten solder will be resident on the preceding electrodes Ei than that on the following electrode Ei+1. In this sense, there is a tendency for preceding pairs of electrode E and lead L to not have enough of molten solder to form the solder mount M. Since any electrode Ei except the first electrode E1 is supplied with the molten solder conveyed from the preceding electrodes Ei-1, such shortage of molten solder can be compensated. However, it is to be noted that the first electrode E1 has no preceding electrode which can supply the solder.
FIG. 9B shows the bonding condition of the first three respective pairs of leads L1 to L3 and electrodes E1 to E3 after the soldering operation. As described above, an insufficient amount of molten solder is left on the first lead L1 and electrode E1 for being distributed therebetween. As a result, the first solder mount M1 can not be formed completely to join the lead L1 and electrode E. However, the following two solder mounts M1 and M2 can be formed for joining the leads L2 and L3 and electrodes E2 and E3, respectively.
FIG. 9C shows the final stage of the soldering operation whereat the soldering iron 160 is located over the last three pairs of leads Ln-2, Ln-1, and Ln and electrodes En-2, En-1, and En. The molten solder not used for forming the solder mount Mi is conveyed to the following electrodes Ei+1 for forming the following mount Mi+1, as described above. Therefore, the solder pool 210 resident around the last three electrodes En-2, En-1, and En includes the remaining part of all preceding solder deposits D1 to Dn-1 not used for mount formation. In other words, the molten solder in the solder pool 210 accumulatively increases each time when the solder mount M is formed.
FIG. 9D shows the bonding condition of the last three pairs of leads Ln-2 to Ln and electrodes En-2 to En after the soldering operation. Since there is no further electrode for joining by the solder, such excessive solder can not be conveyed to anywhere. Therefore, all the molten solder included in the solder pool 210 should be used for forming the last mounts Mn, Mn-1, and Mn-2. As a result, at least two of final mounts are essentially connected by the excessive solder, resulting in short-circuits. Note that such a short-circuit between adjacent mounts Mi and Mi+1 by the excessive solder is not limited to the final stage but can be generated at any stage of bonding operation.
As described above, an amount of the solder M formed on each copper electrode E was maintained constant in order to bond the leads L and copper electrodes E in the same shape. According to the conventional arrangement, the solder pool 210 is not yet sufficiently obtained at the front end 220 of the soldering iron 160 when bonding is started. As typically shown in FIG. 8A, the starting lead L sometimes fails to be properly bonded. This is a problem of the prior art at the beginning of bonding.
During bonding, when the front end 220 of the soldering iron 160 continues bonding, an amount of the solder pool 210 formed at the front end 220 of the soldering iron 160 is increased. As typically shown in FIG. 8B, a short-circuit is generated between the adjacent mounts by the excessive solder in the latter stage of bonding.
Further, since the soldering iron 160 keeps moving in touch with the circuit board 130 even after passing the final lead Ln of FIG. 8, the solder pool 210 accumulated at the front end 220 of the soldering iron 160 overflows to drop on the circuit board 130, thereby giving rise to an inconvenient adhesion of unnecessary solder to the circuit board 130.